(1) Field of the Invention
The present invention relates to an arc tube formed in a spiral that broadens out from the middle part towards the end-side, a low-pressure mercury lamp, a lighting apparatus, a mandrel, and to a production method of such an arc tube.
(2) Related Art
Conventionally, there are fluorescent lamps for general lighting purpose that use a circular arc tube (hereinafter, this type of fluorescent lamp is referred to as “circular fluorescent lamp”). The majority of such circular fluorescent lamps have a rated lamp wattage of 18 w to 40 w, and their usage is varied.
Recently, there are strong demands for making smaller circular fluorescent lamps. This is because if a circular fluorescent lamp is made smaller, the lighting apparatus to which the fluorescent lamp is to be fixed can also be made smaller.
One kind of arc tube smaller than conventional circular arc tube is disclosed in the specifications of West German Patent No. 871927, and of West German Patent No. 860675. The disclosed arc tube is formed by winding a glass tube from its center to the both ends around the axis of spiral (hereinafter, “spiral axis”), into a double-spiral configuration that broadens out from the center to the both ends.
Specifically, a body of the double-spiral arc tube is formed by winding a glass tube in softened state on double-spiral grooves cut on a circumferential surface of a mandrel having a substantially circular-cone shape(hereinafter, “glass tube in softened state” is referred to as “softened glass tube” in this specification). The grooves at the circumferential surface of the mandrel are cut to be concave in a direction orthogonal to the axis of the mandrel, and towards the axis. Furthermore the grooves are formed to be concave in a semicircular shape so as to be in contact with half of the circumference of the glass tube. Note that the arc-tube body formed in double-spiral configuration using this mandrel will be shaped so that its glass tube is wound around the axis of the mandrel, and this axis will be the spiral axis for the arc-tube body and of the arc tube.
The pitch of the grooves in the direction into which the axis of the mandrel extends (this direction is called “height direction”) is larger than the outer diameter of the glass tube wound on the mandrel. Therefore, there is a problem that, when the arc-tube body is viewed in the direction orthogonal to the spiral axis, space exists between adjacent glass tubes in the direction into which the spiral axis extends (this direction is also called “height direction”).
To be more specific, a conventional circular fluorescent lamp is usually fixed to a lighting apparatus having been fixed to the ceiling and the like, and so it is preferably thin. As opposed to this, the arc tube formed using the aforementioned mandrel has large height (i.e. thickness), although being able to have smaller outer diameter than conventional arc tubes used for circular fluorescent lamps. This makes it difficult to make such an arc tube to replace the conventional circular fluorescent lamp.
In addition, suppose that the glass tube's cross section is divided into two by a height-direction line passing through the center of the cross section. Then the cross-section of the grooves of the mandrel has such a shape as to be in contact with a half of the circumference of the glass tube cross-section that is positioned closer to the axis of the mandrel. Accordingly, when taking the wound glass tube out from the mandrel, it is necessary to rotate either the mandrel or the wound glass tube in the reverse direction to the direction of rotation adopted at the time of winding the glass tube. This makes the production of the arc-tube body troublesome, as well as necessitating a complicated production apparatus that includes a driving apparatus for rotating such mandrel or glass tube.